Electric cable



March 3, 1942. s. CLARK ETAL 2,275,187

ELECTRIC CABLE Filed May 13, 1941 /05 5H5! TH IOZ TAPE INSULA T/ON l0! CONDUCTOR flyumrm 801M158 2 2 ATTORNEY Patented Mar. 3, 1942 nmc'rmc can: Gordon Sterling can, Yonkers, and john Hunter Palm er, Scandale, N. Y., assignors to Phelps Dodge Copper Products Corporation, Dover, DeL, a corporation of Delaware Application May 1:, 1941, Serial No. 393,154

3 Claims. (emu-102) This invention relates to electric cables having a taped form insulation and has'for its object to lay the tapes so as to produce a cable having a maximum dielectric strength.

We have discovered that the maximum dielectric strength of a paper tape wrapped conductor is dependent upon staggering the tapes in their lay in such a manner that there will be a small number of radially aligned channels in the straight-through path combined with a long zigzag path.

Dielectric strength tests have shown that insulation composed of layers of tape, breaks down along the weaker of two possible paths. One path is the zig-zag or stepped one around the edgesof the tapes and between adjacent layers; the other is straight-through the tapes. There are, of course, many combinations of these types of paths often present in one failure. In either type of path, the dielectric strength depends to a large extent upon the length possible of path through the dielectric and the amount of the dielectric in the path. However, the-spacing of tapes giving the longest zig-zag path gives the least dielectric in the straight-through path due to lining up of the spaces between adjacent turns of tape. Also, the spacing giving the strongest straight-through path gives the shortest zig-zag path.

In the manufacture of impregnated-paper-tape insulated cable, great effort has been expended to improve the paper taping machines to give uniformity and precision to the spacing of adjacent tapes. Also, it has been determined theoretically that a certain uniform stagger between adjacent tape layers gives a reasonably long zigzag path with a very-strong straight-through the center of the adjacent layers.

' We propose, therefore, to combine a long zig-zag path with a straight through path having a minimum number. of radially aligned channels, by

staggering the tapes non-uniformly. For in- 5 stance, the following system of staggering is shown in Figure 1:

Tape No. I0 20 30 40 50 60 Ill ll 90 Percent stagger--25 50 '75 50 25 50 75 50,

etc.

For the purpose of this specification we define stagger" as follows: "Stagger" is the ratio (l/L) in Figure 1. This is expressed in percent. Where L is the lay of the tape and 1 is the offset between adjacent tapes. All dimensions should be measured parallel to the axis of the cable conductor, and when progressing through the insulation should be measured consistently in one direction.

The foregoing and other features of our invention will now be described in connection with the accompanying drawing forming partof this specification in which we have illustrated our insulation in its preferred form after which we shall point out in the claims those features which we believe to be new and of our own joint invention.

In the drawing: The figure is a longitudinal view of a piece of our cable broken away to show the relative positions of the stepped tapes in the layers.

In the carrying out of our invention we insulate any conductor liil with tape insulation I02 and cover all with a protective covering or sheath I03.

We propose to lay the tape form insulation non-uniformly so that there will be a minimum of radially aligned channels such as 42 and 82 along the line 0-D and with a maximum length of the zig-zag path A. B. To accomplish this the first tape in is laid upon the conductor in a helical laywith the edge i I forming an optimum width helical channel l2 along the length of the conductor. The second tape 2il-is laid upon the first tape with a 25% overlap, so that the channel 22 is offset the channel l2 by A the lay of the tape all along the length of the conductor.

The third tape 20 is laid upon the second tape 20 so that the channel 321s offset the channel 22 by the lay of the tape 20. The fourth tape I0 is laid upon the third tape 30 so that the channel 421s offset the channel 32 by of the lay of the tape 42, and so on. It is important that all .the calculated offsets are measured parallel to 0 nels l2 and 82 to the non-uniformity of the staggers, that is, the 25 and 75% staggers which are alternated with the staggers.

.Such a system of staggeringis found to be a decided improvement over the uniform stagger previously used.

This system has the advantage or combining- (1) Staggering the channels, (2) Giving barrier action against the step or zig-zag failure by inserting the long.

leakage path of tapes with 50% stagger.

We wish it distinctly understood that our cable herein described and illustrated is in the form in which we desire to construct it and that changes or variations may be made as may be convenient or desirable without departing from the salient features of our invention and we therefore intend the following claims to cover such modifications as naturally fall within the lines of inwith a straight-through path in which the channels of any one layer are in radial alignment with a smallpercentage o! the channels of any of the other tapes, and those which are in alignment being approximately uniformly spaced radially through the insulation.

2. An electric cable comprising a metallic conductor, a tape form of insulation laid on the conductor with the open spaces or channels between the turns of the tapes staggered in the order of 2550'755025-50-'75, etc., from the conductor outwardly, a sheath over all.

3. An electric cable comprising a metallic conductor and a multiple tape form of insulation having open spaces or channels between successive turns of each layer, the channels in a group of successive layers being laid with a series of nonuniform staggers and the series repeated as often as required to form the complete insulation.

GORDON STERLING CLARK. JOHN HUNTER PALMER. 

